By way of example, in “Diesel Nozzle—The Determining Interface between Injection System and Combustion Chamber”, THIESEL 2000, Thermofluidynamic Processes in Diesel Engines, pages 249 to 258, D. Potz et al. describe a method of producing an injection hole in a fuel injector wherein the hole is initially produced by an electro discharge machining (EDM) process during which part of the fuel injector is heated by spark discharge and the vaporized material is flushed away by a dielectric fluid. The size and shape of the hole are essentially defined by this process. The EDM process generally takes over 40 seconds to produce the hole with the correct geometry and a low dimensional tolerance. A rounding process (hydro-erosive grinding) is then performed in which a medium containing abrasive particles is forced through the hole.
If the rounding process is of short duration, the edges of the hole opening are rounded. This short rounding process is generally used for producing holes in order to pre-empt aging processes. If the rounding process is performed for somewhat longer, the surface roughnesses of fuel injector surfaces forming the hole are smoothed. Such surface roughnesses typically exhibit height differences of up to 3 μm. An even longer rounding process can increase the hole diameter somewhat, thereby enabling the required flow rate of fuel through the hole during operation of the fuel injector to be finely adjusted. The flow rate is the quantity of fuel flowing through the hole per unit of time at a certain temperature and at a certain pressure difference. However, if the rounding process is performed for longer still, this results in uncontrolled enlargement of the hole causing the symmetry of the hole to be destroyed. The hole then has major geometrical defects.